What frame size for 15 x 15 pictures?

I'm getting ready for a show in a gallery and am wondering about a frame size for 15 x 15 inch prints. I have always printed 10 x 10 in rectangular 16 x 20 frames. I typically optically center the picture. Now I want to move up one size in paper. I use the Optical Print Centering website to help me previsualize this. To me, it looks like I need to have top and sides equal at the larger print dimensions. Also, the top and side borders look a little small (2.5 inches). The bottom is 6.5 inches. Any suggestions would be welcome. Is the propsed framing adequate, or do I really need to consider moving up one more step in frame size?

Not sure I totally follow this; you won't be able to reasonably put a 15x15 in a 16x20 frame (assuming all dimensions are the same units). I might be inclined to go about 22 x 22 with a square frame or maybe 22 x 24 with the 24 vertical and the print optically centered for a little "weight" on the bottom.

I have mounted a lot of 15x15 on 20x24, and it works okay. The next standard size up, 22x28, is nicer, but I find it a bit long for the width. For 16x20 I prefer 24x28 -- might be over-kill for 15x15, but then again, it might give the images a nice formal presentation with 4.5" sides, 5" (or 5.5") top and 8" (or 7.5") bottom.

Take a look again at how you would center on the mat -- consider having more space on top than on the sides. I think that works fine for square images on vertical rectangles.

At least with LF landscape, a bad day of photography can still be a good day of exercise.

I just had a look at 15x15 print in a 24x28 frame, it looks very nice. The 4.5 inch side borders are much better than 2.5 (which make the print look squeezed in). Also, at this frame dimension optically centered does look better than equal top and sides. The 24x28 frame is over 8 feet of frame though! The 22 x 24 looks OK as well. I think I'll buy the Neilsen frames wholesale and cut my own this time.....

I'd almost go for 30x24 the 30" vertical, but that's quite large frames I do use a few for my own exhibitions. I guess a touch smaller is better for you 20x25 or 22x27.7 are the same ratios but either would be custom frames